This invention relates to the production of higher hydrocarbons from synthesis gas by the Fischer-Tropsch process.
The production of higher hydrocarbon materials from synthesis gas, i.e. carbon monoxide and hydrogen, commonly known as the Fischer-Tropsch process, has been in commercial use for many years. In such processes, the synthesis gas mixture is contacted with a suitable Fischer-Tropsch catalyst under shifting or non-shifting conditions, preferably the latter wherein little or no water gas shift takes place. Suitable Fischer-Tropsch catalysts comprise one or more Group VIII catalytic metals, such as iron, cobalt and nickel.
There exist many variations of the basic preparation of Fischer-Tropsch catalysts such as, for example, deposition of alloys onto a performed support by flame spraying, (U.S. Pat. No. 4,089,812), formation of the alloy by surface diffusion of aluminum on a non-leachable metal substrate (U.S. Pat. No. 2,583,619), and forming pellets from the powdered alloys for use in fixed bed reaction vessels (U.S. Pat. Nos. 4,826,799, 4,895,994 and 5,536,694, for example). The choice of a particular catalyst formulation, method of fabrication and method of activation depends in large measure on the catalytic activity, the desired product or products, whether or not the catalyst can be regenerated and the specific process components and configurations.
The production of hydrocarbons by the Fisher-Tropsch process may be carried out in virtually any type reactor, e.g. fixed bed, moving bed, fluidized bed, slurry, bubbling bed and the like. A preferred reactor carrying out such reactions is the slurry bubble column developed by Exxon Research and Engineering Company. This reactor, which is ideally suited for carrying out highly exothermic, three-phase catalytic reactions, is described in U.S. Pat. No. 5,348,982. In such reactors, the solid phase catalyst is dispersed or held in suspension in a liquid phase by a gas phase that continuously bubbles through the liquid phase. The catalyst loading in slurry bubble reactors can vary within a broad range of concentrations, but must remain short of the so-termed xe2x80x9cmud limitxe2x80x9d where the concentration of the catalyst reaches a level such that mixing and pumping of the slurry become so difficult as to render practical operation impossible. The use of high metal-loading catalysts or bulk metal catalysts is preferred in slurry bubble reactors in order to maximize the productivity of both catalyst and reactor.
Particularly suited for the production of hydrocarbons by Fischer-Tropsch synthesis from synthesis gas are Dispersed Active Metals (xe2x80x9cDAMxe2x80x9d) which are primarily, i.e. at least about 50 wt. %, preferably at least 80 wt. %, composed of one or a mixture of metals such as described above and are, without further treatment, capable of catalyzing Fischer-Tropsch synthesis. DAM catalysts may be prepared by any of a number of art-recognized processes. An extensive review of processes of forming DAM catalysts can be found in xe2x80x9cActive Metalsxe2x80x9d, Edited by Alois Furstner, published by VCH Verlagsgesellschaft mbH, D-69451 Weinheim (FRG) in 1996 and the references cited therein. Methodologies described therein include the Rieke method, the use of ultrasound, reduction of metal salts, colloids, nanoscale cluster and powders. Other relevant references include, for example, the preparation of amorphous iron catalyst by high intensity sonolysis of iron pentacarbonyl, Suslick et al., Nature, Vol. 353, pp. 414-416 (1991) and the formation of single domain cobalt clusters by reduction of a cobalt salt with hydrazine, Gibson et el., Science, Vol. 267, pp. 1338-1340, (1998). Finally, intermetallic alloys, particularly those known for forming metal hydrides, such as LaCo5, can be formed into a fine powder by the application of hydrogen adsorption/desorption cycles. DAM catalysts can also be prepared by thermal or chemical decomposition of metal formates or oxalates. These methods are given as examples and are not intended in any way to limit the term xe2x80x9cDAMxe2x80x9d as utilized in the context of the present invention.
There are many well-known methods for the preparation of DAM catalysts in the literature. In 1924, M. Raney prepared a Nickel hydrogenation catalyst by using a process known today as the Raney Process and Raney catalysts. Such catalysts are described and illustrated, for example, in U.S. Pat. No. 4,826,799. The process of preparing these catalysts is, in essence, forming at least a binary alloy of metals, at least one of which can be extracted, and extracting it leaving a porous residue of the non-soluble metal or metals that possesses catalytic activity. These groups of metals are well known to those skilled in the art. The residue catalyst metals include Ni, Co, Cu, Fe and the Group VIII noble metals. The leachable or soluble metal group includes aluminum, zinc, titanium or silicon, typically aluminum. Once the alloys are formed, they are ground to a fine powder and treated to extract the leachable metal, typically with strong caustic, such as sodium hydroxide. Alternatively, the alloy is formed onto or impregnated into a suitable rigid support structure which is then extracted with caustic to form a porous, supported catalyst.
The high metal content of DAM catalysts, i.e. at least 50% metal, represents a major economic impediment to their use unless low cost recovery technology can be implemented as well. Those of ordinary skill in the art are aware that metals constituting DAM catalysts, particularly Raney catalysts, are conventionally recovered by subjecting the used, or spent, catalysts to multiple processing steps, principally for the purpose of purification of the metal. The particular methodology chosen to purify and recover the metal depends in large measure on the nature of the impurities and contaminants that have been deposited on the catalyst during use. In most applications, drastic treatments are required because of significant contamination of the metals by one or more of carbonaceous deposits, heteroorganic compounds, i.e. compounds containing sulfur and/or nitrogen, and other metals.
Typically, spent DAM catalysts are treated in the reactor by oxidation to permit safe unloading and shipping to a metal processing facility. The oxidation can be carried out, for example, by air oxidation of the catalyst slurry, or by treatment with bleach as recommended by catalyst manufacturers. In the metal processing facility, the catalysts are generally roasted in air, dissolved in strong acid and the different metals selectively reprecipitated in the form of salts. The metals may be reused in the form of the salts, or converted back into metallic form, depending on the requirements of the synthesis. Such treatments must be effective and efficient because, although carbon monoxide hydrogenation processes are conducted in an exceptionally clean environment, DAM catalysts are generally sensitive to comparatively minor amounts of contaminants.
Those of ordinary skill in the art recognize that the economic worth of a given catalyst is a function of its original cost, its value as a spent catalyst, e.g. for regeneration of fresh catalyst, its activity and its half-life in the reactor. Another important aspect of the value of a catalyst is its selectivity which is the ratio of the percent of feed material converted to desired higher hydrocarbons to that of short chain hydrocarbons produced, primarily methane, commonly referred to as xe2x80x9cmethane selectivityxe2x80x9d. It will be appreciated that a process that will effectively extend the useful life of a catalyst before it must be disposed of through conventional metal recovery will significantly improve the value of that catalyst. In accordance with the present invention, an improved Fischer-Tropsch process is provided wherein a plurality of reactors is utilized to process incoming synthesis gas with enhanced efficiency in terms of the activity and methane selectivity of a catalyst and, therefore, the overall efficiency of the process.
In accordance with the present invention, Fischer-Tropsch synthesis of higher hydrocarbons from synthesis gas is carried out in a system comprising a plurality of reactors operably connected in series comprising at least one initial stage reactor and a final stage reactor, wherein the catalyst in the at least one initial stage reactor is periodically or continuously renewed by withdrawing a mixture of hydrocarbons and a portion thereof, reducing the hydrocarbon content of the mixture, forming a melt by heating to the melting temperature of at least one of the metals in the catalyst, removing any slag that forms on the melt, cooling the melt to solidify it, reducing the particle size of the solid to a fine powder catalyst and returning at least a portion thereof to the at least one initial stage reactor. Wherein the catalyst in the at least one initial stage reactor is a Raney catalyst, a leachable metal is added to either the reduced hydrocarbon catalyst mixture or the melt and the solid formed by cooling the melt is comminuted to a fine powder and then extracted with caustic.
The renewed catalyst may be treated to further enhance its activity and selectivity by slurry low temperature oxidation and may also be passivated before being returned to the at least one initial stage reactor. The treatment of the catalyst in the at least one initial stage reactor according to the subject process permits a maximum productivity catalyst in the final stage reactor to operate at high efficiency for extended periods of time because the impurities it would ordinarily be exposed to are minimized. Depending on the catalyst in the final stage reactor, the catalyst withdrawn from the at least one initial stage reactor and renewed may be at least partially fed into the final stage reactor. A DAM catalyst in the final stage reactor may be enhanced by slurry low temperature oxidation and either replaced into the final stage reactor or recycled into the at least one initial stage reactor. In a further embodiment, there is at least one intermediate stage reactor that may receive renewed catalyst from the at least one initial stage reactor and recycle catalyst thereto. Wherein the catalyst in the final stage reactor is a DAM catalyst, further embodiments encompass variations for catalyst renewal, enhancement and recycle.